Shadow Casts

δεν ήξερα που να την ανοίξω, στο θέατρο ή στον κινηματογράφο, αν κρίνετε οτι ταιριάζει στο θέατρο καλύτερα μεταφέρετε το

The gold standard. The audience at a showing of Rocky Horror will have some sort of response (mostly yelled comments, sometimes actions to match) to virtually every line or action in the film; at this point the cult phenomenon of Rocky Horror is far more about the audience participation than the film itself. Most theaters will give you a bag of items to be used during the movie (most getting thrown).

Inverted audience participation. Where the cast will make callbacks to the audience's callbacks.
When they tried to stage the musical in the US, of course the audience yelled the callbacks. Apparently, in the first performance, they were studiously ignored until Frank N Furter got to "antici..." "SAY IT!" The actor then snapped back with "bait me, why don't you?", and promptly continued on with the song.
Furthermore, new callbacks are invented all the time. A Halloween 2010 viewing added "Ten points from Gryffindor!" after Frank admonished Rocky that "That's no way to behave after your first day out!"
Likewise, after Fight Club, a popular call back following the revelation of Eddie's corpse has been, "HIS NAME WAS ROBERT PAULSON! HIS NAME WAS ROBERT PAULSON!"
Many older callbacks get updated references but otherwise remain the same. For example, when Riff-Raff shows Brad and Janet the skeleton near the start of the film, it's customary to shout out "SHOW US" and the name of whatever celebrity has most recently died. Bonus points if it's someone people really liked. (For Halloween 2011 and shows thereafter, a popular one is "SHOW US STEVE JOBS!")
Take Thats are common, usually involving Magenta. "DESCRIBE JUSTIN BIEBER'S PERIOD COLOR!" "Magenta..." Some get fired back at the audience: "WHAT THE FUCK IS AN 'UNT'?" "THREE FOURTHS OF A CUNT! ...And that's more than you'll ever see!"
For this reason, it's a good idea to watch the movie once by yourself before you go to a midnight screening—because at the midnight screening, you may not be able to hear any of it. The (extremely loud) audience commentary is not funny unless you've had the opportunity to actually hear the dialogue and music at some point.

On the other hand, in a venue where the film has been running for long enough to develop a crowd of regulars as well as a live cast of reenactors, the discipline of the audience can be astonishing. It truly does seem like an interactive experience.

The shadowcast productions of this show encourage this to varying degrees. Exactly how they do this varies depending upon the cast.
Most shadowcasts encourage the audience to stand up and sing during certain key songs, particularly We Started This Op'ra Shit!
It is also common to have the audience read along with text in the comic-book exposition sequences.

Various props are waved about or thrown during the show. These vary from cast to cast but generally include:
Panties - thrown when Pavi snatches the Panties from the Genterns and throws them at Luigi.
Neon Blue Glowsticks - waved during Zydrate Anatomy.
Toy Gold Coins - thrown at Rotti as he sings Gold.
Tampons - thrown at Shilo after she collapses and wakes up in a pool of blood at the start of Shilo Turns Against Rotti.

There's also a number of Mystery Science Theater 3000-style callbacks, which vary from cast to cast. For example:
Doctor: "It's spreading rapidly."
AP: "Like Amber's legs!"
Also:
Graverobber: "It's the 21st Century..." (Beat)
AP: (during the (Beat) "What's your favorite Goth band?!"
Graverobber: "...Cure!"
Or:
(Blimp advertising Opera flies past)
AP: Start a new life on the Off-World Colonies! Drink Coke!